MCMS Site Deployment API does not have a method to export Resource Galleries. Only resources (and related galleries) which are bound to an exported posting can be exported using the API.
A workaround for this is to create a helper posting that links to all resource gallery items which should be exported. After the export is finished the helper posting is no longer required and can be deleted.
I have written a code sample that can automate this process.
Ever had a need to export resources from a resource gallery using script? Then you will have seen that...